This invention relates to condensers that collect radiation and deliver it to a ringfield. More particularly, this condenser collects radiation, here soft x-rays, from either a small, incoherent source or a synchrotron source and couples it to the ringfield of a camera designed for projection lithography.
In general, lithography refers to processes for pattern transfer between various media. A lithographic coating is generally a radiation-sensitized coating suitable for receiving a projected image of the subject pattern. Once the image is projected, it is indelibly formed in the coating. The projected image may be either a negative or a positive of the subject pattern. Typically, a xe2x80x9ctransparencyxe2x80x9d of the subject pattern is made having areas which are selectively transparent, opaque, reflective, or non-reflective to the xe2x80x9cprojectingxe2x80x9d radiation. Exposure of the coating through the transparency causes the image area to become selectively crosslinked and consequently either more or less soluble (depending on the coating) in a particular solvent developer. The more soluble (i.e., uncrosslinked) areas are removed in the developing process to leave the pattern image in the coating as less soluble crosslinked polymer.
Projection lithography is a powerful and essential tool for microelectronics processing. As feature sizes are driven smaller and smaller, optical systems are approaching their limits caused by the wavelengths of the optical radiation. xe2x80x9cLongxe2x80x9d or xe2x80x9csoftxe2x80x9d x-rays (a.k.a. Extreme UV) (wavelength range of xcex=100 to 200 xc3x85 (xe2x80x9cAngstromxe2x80x9d) are now at the forefront of research in efforts to achieve the smaller desired feature sizes. Soft x-ray radiation, however, has its own problems. The complicated and precise optical lens systems used in conventional projection lithography do not work well for a variety of reasons. Chief among them is the fact that there are no transparent, non-absorbing lens materials for soft x-rays and most x-ray reflectors have efficiencies of only about 70%, which in itself dictates very simple beam guiding optics with very few surfaces.
One approach has been to develop cameras that use only a few surfaces and can image with acuity (i.e., sharpness of sense perception) only along a narrow arc or ringfield. Such cameras then scan a reflective mask across the ringfield and translate the image onto a scanned wafer for processing. Although cameras have been designed for ringfield scanning (e.g., Jewell et al., U.S. Pat. No. 5,315,629, Offner, U.S. Pat. No. 3,748,015, and Hudyma et al, U.S. Pat. Nos. 6,262,836 and 6,318,869), available condensers that can efficiently couple the light from a synchrotron source to the ringfield required by this type of camera have not been fully explored. Furthermore, full field imaging, as opposed to ringfield imaging, requires severely aspheric mirrors in the camera. Such mirrors cannot be manufactured to the necessary tolerances with present technology for use at the required wavelengths.
The present state-of-the-art for Very Large Scale Integration (xe2x80x9cVLSIxe2x80x9d) involves chips with circuitry built to design rules of 0.13 xcexcm. Effort directed to further miniaturization takes the initial form of more fully utilizing the resolution capability of presently-used ultraviolet (xe2x80x9cUVxe2x80x9d) delineating radiation. xe2x80x9cDeep UVxe2x80x9d (wavelength range of xcex=0.3 xcexcm to 0.1 xcexcm), with techniques such as phase masking, off-axis illumination, and step-and-repeat may permit design rules (minimum feature or space dimension) of 0.10 xcexcm or slightly smaller.
To achieve still smaller design rules, a different form of delineating radiation is required to avoid wavelength-related resolution limits. One research path is to utilize electron or other charged-particle radiation. Use of electromagnetic radiation for this purpose will require x-ray wavelengths. Various x-ray radiation sources are under consideration. One source, the electron storage ring synchrotron, has been used for many years and is at an advanced stage of development. Synchrotrons are particularly promising sources of x-rays for lithography because they provide very stable and defined sources of x-rays, however, synchrotrons are massive and expensive to construct. They are cost effective only when serving several steppers.
Another source is the laser plasma source (LPS), which depends upon a high power, pulsed laser (e.g., a yttrium aluminum garnet (xe2x80x9cYAGxe2x80x9d) laser), or an excimer laser, delivering 500 to 1,000 watts of power to a 50 xcexcm to 250 xcexcm spot, thereby heating a source material to, for example, 250,000xc2x0 C., to emit x-ray radiation from the resulting plasma. LPS is compact, and may be dedicated to a single production line (so that malfunction does not close down the entire plant). The plasma is produced by a high-power, pulsed laser that is focused on a metal surface or in a gas jet.
Discharge plasma sources have been proposed for photolithography. Capillary discharge sources have the potential advantages that they can be simpler in design than both synchrotrons and LPS""s, and that they are far more cost effective. This source is capable of generating narrow-band EUV emission at 13.5 nm from the 2-1 transition in the hydrogen-like lithium ions. Another source is the pulsed capillary discharge source which is expected to be significantly less expensive and more efficient than the laser plasma source.
Projection lithography has natural advantages over proximity printing. One advantage is that the likelihood of mask damage is reduced, which reduces the cost of the now larger-feature mask. Imaging or camera optics in-between the mask and the wafer compensate for edge scattering and, so, permit use of longer wavelength radiation. Use of extreme ultra-violet radiation (a.k.a., soft x-rays) increases the permitted angle of incidence for glancing-angle optics. The resulting system is known as extreme UV (xe2x80x9cEUVLxe2x80x9d) lithography (a.k.a., soft x-ray projection lithography (xe2x80x9cSXPLxe2x80x9d)).
A favored form of EUVL is ringfield scanning. All ringfield optical forms are based on radial dependence of aberration and use the technique of balancing low order aberrations, i.e., third order aberrations, with higher order aberrations to create long, narrow illumination fields or annular regions of correction away from the optical axis of the system (regions of constant radius, rotationally symmetric with respect to the axis). Consequently, the shape of the corrected region is an arcuate or curved strip rather than a straight strip. The arcuate strip is a segment of the circular ring with its center of revolution at the optic axis of the camera. See FIG. 4 of U.S. Pat. No. 5,315,629 for an exemplary schematic representation of an arcuate slit defined by width, W, and length, L, and depicted as a portion of a ringfield defined by radial dimension, R, spanning the distance from an optic axis and the center of the arcuate slit. The strip width is a function of the smallest feature to be printed with increasing residual astigmatism, distortion, and Petzval curvature at distances greater or smaller than the design radius being of greater consequence for greater resolution. Use of such an arcuate field allows minimization of radially-dependent image aberrations in the image. Use of object:image size reduction of, for example, 5:1 reduction, results in significant cost reduction of the, now, enlarged-feature mask.
It is expected that effort toward adaptation of electron storage ring synchrotron sources for EUVL will continue. Economical high-throughput fabrication of 0.13 xcexcm or smaller design-rule devices is made possible by use of synchrotron-derived x-ray delineating radiation. Large angle collection over at least 100 mrad will be important for device fabrication. Design of collection and processing optics for the condenser is complicated by the severe mismatch between the synchrotron light emission pattern and that of the ringfield scan line.
Sweatt, U.S. Pat. No. 5,512,759, discloses a condenser for collecting and processing illumination from a synchrotron source and directing the illumination into a ringfield camera designed for photolithography. The condenser employs a relatively simple and inexpensive design, which utilizes spherical and flat mirrors that are easily manufactured. The condenser employs a plurality of optical mirrors and lenses, which form collecting, processing, and imaging optics to accomplish this objective.
Sweatt, U.S. Pat. No. 5,361,292, discloses a condenser that includes a series of aspheric mirrors on one side of a small, incoherent source of radiation. If the mirrors were continuously joined into a parent mirror, they would image the quasi point source into a ring image with a diameter of a few tens of centimeters at some distance, here some number of meters. Since only a relatively small arc (about 60 degrees) of the ring image is needed by the camera, the most efficient solution is to have about five 60xc2x0 beams, all of which are manipulated such that they all fall onto the same arc needed by the camera. Also, all of the beams must be aimed through the camera""s virtual entrance pupil. These requirements are met in two steps.
First, the beams are individually rotated and translated, as necessary, using mirrors so that they overlap at the ringfield and pass through the real entrance pupil without interfering with each other. The second step is to image this real entrance pupil into the camera""s virtual entrance pupil using a powered imaging mirror. This places the corrected, combined images of the mirrors into the proper position for use by the camera. This system may be configured in a variety of ways.
Despite the advantages in condenser designs for projection lithography, the art is in search of techniques that enhance critical dimension (CD) control.
Condenser designs such as those disclosed by Sweatt are quite efficient in that they collect about 1 steradian of light from a small point incoherent source or 20xc2x0 or 30xc2x0 from a synchrotron and pass it through the ringfield and into the entrance pupil. However, the illumination pattern in the entrance pupil is far from uniform causing a large CD error.
The 3-nm CD error typically obtained with 100-nm features using current techniques is quite large and requires that the masks be designed and then modified to remove these predictable errors. This mask modification step is an iterative procedure where the mask geometry corrections are estimated and then the corrected mask must be modeled. This step adds expense and uncertainty to the function of the completed chips, even possibly requiring a redesign and the fabrication of a new set of masks.
Although the above described condensers are commercially viable, each of the aforementioned condensers is limited in its ability to change coherence of the radiation at the entrance pupil of the camera without changing the configuration of the mirrors within the condenser. Furthermore, each of the aforementioned condensers is limited in its ability to engineer the illumination of the source image.
It is an object of the present invention to overcome one or more limitations of the prior art hereinabove enumerated. It is another object of the present invention to provide a condenser that can change coherence of radiation at the entrance pupil of the condenser. It is a further object of the present invention to provide a condenser in which the source image may be engineered.
Furthermore, another object of the present invention is to allow the intensity at the mask to be varied, for example, to make it uniform to the limits of measurement.
In one embodiment, the invention is directed to a condenser for a photolithography system wherein a mask image from a mask is projected onto a wafer through a camera having an entrance pupil, said condenser including:
a source of propagating radiation;
a first mirror illuminated by said radiation;
a mirror array illuminated by said radiation reflected from said first mirror, said mirror array including a plurality of micromirrors, each of said mirrors being selectively actuatable independently of each other; and
a second mirror illuminated by said radiation reflected from said array, said first mirror and said second mirror being disposed such that said source is imaged onto a plane of said mask and said mirror array is imaged into said pupil.
In another embodiment, the invention is directed to a photolithography system wherein an image of a mask is projected onto a wafer that includes:
a source of propagating EUV radiation;
a first mirror illuminated by said radiation;
a mirror array illuminated by said radiation reflected from said first mirror, said mirror array including a plurality of micromirrors, each of said mirrors being selectively actuatable independently of each other;
a second mirror illuminated by said radiation reflected from said array; and
a camera operable in a wavelength of said radiation and having an entrance pupil, said first mirror and said second mirror being disposed such that said source is imaged onto a plane of said mask and said mirror array is imaged into said pupil.
A feature of the present invention is that the partial coherence or the illumination in the entrance pupil can be changed on the fly. Another feature of the present invention is that the illumination at a slit in the mask plane may be engineered, for example, made uniform across the mask.
The present invention is advantageous over the prior art in that the number of mirrors (counting the array as a single mirror) used within the condenser is reduced. This increases the condenser""s transmission so less source power is needed.